Hey, I'm generally in the "mind your own business" camp, but there are times when it's not a terrible idea to say something.

For example, there are a lot of dudes in the city that like to fly through red lights, including some close passes between the pedestrians that are crossing at the lights. If I happen to catch up to them later on (because they generally aren't very fast when simply riding), I usually let them know that they are simply breeding resentment for all riders. Or, as more common phrased, they are making the rest of us look like a-holes too...